_complete_help fails in some contexts

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

This occurs for example when pressing ^Xh immediately after:

% : ${()

In this specific case it seems to be because _parameters never calls
_describe, so never goes through the _tags loop, which _complete_help
relies upon to populate its results.  It calls _description directly
(instead?).

On cursory inspection, most completion functions do one of:  call
_describe; call both _tags and _description; or call _description but
then call another function (such as _files) that does one of those
first two.

I haven't come up with a simple way to find examples other than
_parameters that never manage to call _tags at all.